1. Field of the Invention
The present invention relates to a two-cycle internal combustion engine, and in particular to a two-cycle internal combustion engine of the fuel injection type, which is relatively small in size and suited for use in a portable working machine such as a chain saw or a bush cutter.
2. The Prior Art
The two-cycle internal combustion engine of relatively small size (for example, total displacement: 21.2 cc; cylinder bore: 32.2 mm; and piston stroke: 26 mm), which is now employed in a portable working machine such as a chain saw or a bush cutter is generally constructed as illustrated in FIG. 4. Namely, the two-cycle internal combustion engine 1' shown in FIG. 4 includes a cylinder portion 2' having a cylindrical inner wall in which a piston 4' is movably inserted, a head portion 7' provided with a so-called squish dome-shaped combustion chamber 3' and formed integrally with the cylinder portion 2', a plurality of air-cooling fins 8' formed around the outer peripheral portions of the cylinder portion 2' and the head portion 7', and a split type crankcase 5' attached to the lower end portion of the cylinder portion 2' and provided therein with a crank chamber 6'.
The inner peripheral wall of the cylinder portion 2' is provided with a suction port 41' and an exhaust port 40', both facing towards each other and being offset vertically from each other. The inner peripheral wall of the cylinder portion 2' is also provided with a pair of scavenging ports 42', 42' facing towards each other in a direction perpendicular to the facing direction of the ports 40' and 41', thereby forming a two-cycle internal combustion engine of the so-called Schnuerle type.
In order to facilitate the monoblock casting of the cylinder portion 2' and the head portion 7' by means of a high pressure die casting, a pair of openings-for-casting 44', 44' are formed respectively along scavenging passages 43', 43' communicating with each of the scavenging ports 42', 42' thus allowing the outer side of each scavenging passage 43' to be exposed to the outer atmosphere. Accordingly, a pair of scavenging passage covers 45', 45' each having a smoothly curved inner surface in conformity with the scavenging passage 43' and prepared separately from the cylinder portion 2', are to be attached to the openings-for-casting 44', 44' respectively, so as to close the openings-for-casting 44', thereby forming smoothly curved complete passages of the scavenging passage 43'.
The crank chamber 6' is formed of a short cylindrical shape and hermetically closed. A crank shaft 30' is axially held at a central portion of each of the right and left sides of the crank chamber 6'. The piston 4' is connected via a connecting rod 32' to a crank pin 31' of the crank shaft 30'. A pair of sector-shaped crank webs 34', 34' are fixed at the right and left ends of the crank pin 31' so that the connecting rod 32' is interposed between the pair of sector-shaped crank webs 34', 34'. Thus, these crank webs 34', 34' are adapted to be rotated integrally with the crank shaft 30'.
For the purposes of obtaining a predetermined rated horsepower, feeding a required quantity of air-fuel mixture into the combustion chamber 3', and effectively discharging combustion exhaust gas, the two-cycle internal combustion engine 1' is designed as shown in FIG. 3B. Namely, the positional relationships between the scavenging ports 42', 42' and the exhaust port 40', i.e. the positions, shapes and sizes of the scavenging ports 42', 42' and the exhaust port 40', are designed such that the openings of the ports 42', 42' and 40' would be simultaneously overlapped with each other as shown in FIG. 3B. Namely, the lateral lower edges of the exhaust port 40' are contacted with a portion of the bore surface of the combustion chamber 3', which is located at an angle of 53.degree. as measured rightward and leftward from the center 0' of the combustion chamber 3', starting from the central longitudinal sectional line F' dividing the exhaust port 40' into two halves. Thus, the exhaust port 40' is opened to such an extent that the lateral width thereof corresponds to a central angle of 106.degree.. On the other hand, the location of each scavenging port 42' is formed such that it is shifted by an angle of 16.degree. from the lateral edge of the exhaust port 40', and the lateral width of each scavenging port 42' corresponds to an angle of 64.degree. as measured from the center 0' of the combustion chamber 3'.
With an increasing concern of environmental problems in recent years and the trend to further strengthen environmental regulations, a reduction not only of the toxic substances in the exhaust gas but also the decreased engine noise is strongly demanded even in a small two-cycle internal combustion engine which is adapted to be employed in a portable working machine.
In particular, there are problems inherent to a two-cycle internal combustion engine, i.e. problems of how to minimize the noise and vibration resulting from the high speed revolution of the engine, of how to reduce the quantity of HC in the exhaust gas which is brought about due to a phenomenon of blow-by of unburnt air-fuel mixture from the combustion chamber, or of how to prevent the discharge of the unburnt air-fuel mixture which is brought about due to a phenomenon of spitting of fuel toward the air-cleaner. The solution of these problems are now strongly demanded.
Reductions in the weight and manufacturing cost of small two-cycle internal combustion engines are also problems that should be solved. In an attempt to solve these problems, the conventional two-cycle internal combustion engine is generally designed to be operated at a high revolution speed of 8,000/min. or more, in order to make it possible to output a predetermined required horsepower with an internal combustion engine of as small an engine displacement as possible. This high revolution speed may be contrasted with a four-cycle internal combustion engine, which is generally operated at a revolution speed of at most 4,000/min. Namely, due to such a high revolution speed in the operation of a small two-cycle internal combustion engine, the noise and vibration become inevitably large.
Generally, the magnitudes of engine noise and vibration are proportional to the revolution speed of the engine, i.e. the larger the engine revolutions, the larger the magnitudes of engine noise and vibration. Therefore, if the revolutions of a small two-cycle internal combustion engine is made smaller, the magnitudes of engine noise and vibration can be made smaller. However, if the engine revolutions is lowered, the output of the engine is also lowered, so that it would become impossible to obtain a required horsepower.
It may be possible to obtain a required output or horsepower while reducing the engine revolutions, provided that an internal combustion engine of large enough displacement is employed and operated with reduced engine revolutions. However, when the displacement of an internal combustion engine is increased, the entire structure of the internal combustion engine becomes inevitably larger and, at the same time, the gross weight of the engine becomes heavier since parts of higher strength are required to be employed in conformity with an increased output to be obtained. Therefore, if such a heavy engine is mounted on a portable working machine, the ease of manipulability of the working machine deteriorates, and the manufacturing cost is increased. Moreover, since the internal combustion engine is forced to be operated at a low revolution speed without being able to output its inherent potential horsepower, the engine cannot be operated effectively. There is also a danger that the working machine may be unintentionally operated with an excessive output over a predetermined magnitude.